At https://bitcoinfees.21.co/ we find out how much we should send satoshi per byte, on average, for our transactions to be approved.

How do we calculate the total number of bytes in a transaction we have done?

In this case, you're talking about the transaction you "have done".Simply check your transaction using blockchain explorers such as https://blockchain.info/tx/INSERT_TXID_HERE. Once you have done inserting your TXID, you will see some information regarding your transaction and there you can find the size of your transaction.Example:

At https://bitcoinfees.21.co/ we find out how much we should send satoshi per byte, on average, for our transactions to be approved.

How do we calculate the total number of bytes in a transaction we have done?

In this case, you're talking about the transaction you "have done".Simply check your transaction using blockchain explorers such as https://blockchain.info/tx/INSERT_TXID_HERE. Once you have done inserting your TXID, you will see some information regarding your transaction and there you can find the size of your transaction.Example:

No, I want to calculate it before I do it. After the transaction, I can check the sites like blocktrail.com or blockchain.info But what I want to know is knowing how many bytes it is before processing. Then I would like to enter a reasonable fee using the average values at bitcoinfees.21.co